Cutting dies, in particular rotary cutting dies for cutting paper products, are well known. Cutting dies are used for cutting shapes in paper, plastic film and thin metal foil. In particular, they are used in the printing and packaging industries for cutting perforations and openings in pressure sensitive labels.
Pressure sensitive labels, markers, and other similar articles formed by cutting dies are usually made available in strips or rolls. A plurality of such labels or the like are attached to an elongated backing layer by a pressure sensitive adhesive or gum. The pressure sensitive adhesive or gum is permanently adhered to the back of the label while the backing layer is provided with a release coating. Hence, a label having pressure sensitive adhesive separates readily from the backing layer or release layer. The user merely peels the labels or markers from the backing layer and applies the peeled-off labels or markers as required.
In order to mass produce these labels or markers, a strip comprising a layer of label material, a layer of pressure sensitive adhesive and a backing layer are assembled and passed under the cutting die. The cutting die may have any convenient arrangement or array of patterns thereon in accordance with the shapes desired for the labels. The labels are formed by cutting through the layer of label material adhered to the backing layer and through the adhesive layer beneath the layer of label material. Often, the dimensions of the cutting edge, as well as the tolerance of the dimensions, are selected to prevent the die from cutting through the backing layer. In this manner a continuous strip or sheet of labels is maintained.
Furthermore, it is known to form a flexible die or die roll by a chemical etching process. The chemical etching of the flexible dies is performed primarily by the use of a photofabrication technique. In this photofabrication technique the metal forming the die is first coated on its front side with a light sensitive "photo-resist". The photosensitive resist is exposed to ultraviolet rays through a photographic transparency containing a clear image of the features of the die cutting pattern. Flexible cutting dies formed by this type of chemical photoetching process are either secured magnetically to magnetic cylinders or rollers or to non-magnetic cylinders using an adhesive layer between the die and the non-magnetic cylinder.
One particular use for the cutting dies described above is for the postage stamp industry, specifically for the production of "peel-away" stamps which have an adhesive layer that enables the stamp to be peeled from a backing sheet and placed on an envelope or other surface without having to wet the back of the stamp. Generally, the peel-away stamps that are produced by known methods have the serpentine edges which are familiar to the old style gummed-back stamps. However, sheets of peel-away stamps as currently produced have a solid backing sheet which does not allow a backing section for an individual stamp to be easily detachable from the remainder of the backing sheet when detaching a single stamp.
In an attempt to overcome the inability of peel-away stamps to have an individually detachable backing, a die sheet was developed to produce sheets of postage stamps which have a stamp sheet with continuous openings and a backing sheet with spaced openings aligned with the openings of the stamp sheet. These die sheets are referred to as a "perf-over-slit" die sheets and they have two cutting levels: a lower slitting level having continuously joined cutting members and an upper perforating level which has spaced cutting members. With such a die sheet, a pattern of straight lines of spaced openings can be produced in a first sheet of a material and a pattern of straight continuous openings can be produced in a second sheet of material which is in facing relationship with the first sheet of material. Postage stamps have been produced by such dies, but such postage stamps have not been accepted by collectors who have rejected the straight-edged stamps for not being as aesthetically pleasing as classical serpentine-edged stamps.
The utility of perf-over-slit die sheets has been limited to producing patterns of straight cuts in materials because it has been too difficult and too costly to attempt to construct a perf-over-slit die sheet with cutting edges that produce curved openings using conventional methods. Attempts have been made to produce perf-over-slit die sheets by forming a cutting surface extending above a die sheet by a chemical etching process and then machining away sections of the material to produce a lower cutting level (the slitting level) at sections where the material has been machined away and an upper cutting level (the perforating level) at sections where no machining occurred.
As these die sheets have relatively short cutting levels and closely packed die patterns, it has been extremely difficult to attempt to produce perforating sections on a curved cutting pattern due to the large number of direction changes required during the milling or grinding of a curved die pattern. Attempts at producing a die sheet having a curvilinear "perf-over-slit" die pattern using conventional machining methods have been unsuccessful.
Therefore, there is a need for a paper product, particularly for the postage stamp industry, which has a pattern of continuous, serpentine-shaped openings in a first sheet of material (the sheet of stamps) and a pattern of spaced, curvilinear openings in a second sheet of material (the backing sheet) which are aligned with the openings of the first sheet. Such a paper product will have an appearance which appeals to stamp collectors and will enable the individual stamps to be separable from the remainder of the sheet along with an individual section of the backing material. Thus, there is also the need a die sheet to form such a product and a method or methods for producing such a die sheet.